Population Dynamics of Alligator Gar in Choke Canyon Reservoir, Texas: Implications for Management

Greg R. Binion, Texas Parks and Wildlife Department, Inland Fisheries District 1E, P.O. Box 116, Mathis, TX 78368 Daniel J. Daugherty, Texas Parks and Wildlife Department, Heart of the Hills Fisheries Science Center, 5103 Junction Hwy., Mountain Home, TX 78058 Kristopher A. Bodine, Texas Parks and Wildlife Department, Heart of the Hills Fisheries Science Center, 5103 Junction Hwy., Mountain Home, TX 78058

Abstract: Historical eradication efforts, increasing fishing pressure, and growing anthropogenic impacts have resulted in decreased abundance or extirpation of the alligator gar (Atractosteus spatula) throughout much of its historic distribution. Current population status has prompted states to actively manage stocks; however, efforts are hindered by a lack of data necessary to make informed management decisions. To begin addressing these data needs, we investigated alligator gar population dynamics and exploitation in Choke Canyon Reservoir, Texas. A total of 754 fish (total length [TL] range, 678 to 2275 mm) was collected with multifilament gill nets from 2008 through 2013; 656 individuals collected from 2011 through 2013 were tagged and released as part of a mark-recapture study to estimate abundance and exploitation. Alligator gar age ranged from 0 to 27 yrs. Growth in length followed a typical von Bertalanffy function with greatest growth occurring through age 5, then considerably slowing among older age classes. Growth in weight, per unit length, was greatest in fish ≥ 1700 mm TL. Length-at-age of females was significantly greater than male length-at-age; TL of female fish was about 278 mm greater than male fish of the same age. Adult alligator gar (TL ≥ 1100 mm) abundance was estimated at 5437 (95% CI; 3215 to 9195) individuals, conferring a density of 0.5 (0.3 to 0.9) fish ha –1 at conservation pool. Annual exploitation based on tag returns was less than 3%; bowfishing tournament data indicated a bow angler harvest rate of 0.01–0.02 fish angler hour–1. Results of our study provide important information for the management of alligator gar populations in Texas and throughout its distribution.

Key words: Atractosteus spatula, abundance, exploitation, age structure, growth Journal of the Southeastern Associated of Fish and Wildlife Agencies 2:57–63

The alligator gar Atractosteus( spatula) is among North Amer- Florida is declining, and population status in the remaining seven ica’s most ancient, distinctive, and historically disliked fishes states is currently unknown. As a result, the American Fisheries (Scarnecchia 1992). Recreational anglers have long viewed alliga- Society (AFS) recently identified the alligator gar as “vulnerable” tor gar as direct competitors for preferred game fishes, and com- (Jelks et al. 2008). mercial fishers considered their large body size, bony scales, and In contrast to the historic aversion toward alligator gar, attitudes prominent teeth significant threats to fishing gears (O’Connell et and opinions have changed in recent decades. Their large body al. 2007). The unfavorable opinions of anglers were historically size, predatory nature, and distinctive characteristics have become shared by fisheries managers, and efforts to eradicate gars (Lepi- desirable trophy attributes (Scarnecchia 1992), and hook-and-line sosteidae) were undertaken for a century or more (Scarnecchia and bow fishing for alligator gar have become popular (Ross et al. 1992). The success of these removal efforts, coupled with increased 2001). Increased interest in alligator gar angling, as well as support loss of critical habitat in aquatic systems, resulted in decreased for maintaining biodiversity in aquatic ecosystems (O’Connell et abundance or extirpation of alligator gar throughout much of its al. 2007), have resulted in efforts to actively manage alligator gar native range (e.g., Gilbert 1992, Etnier and Starnes 1993, Ross et al. populations. Seven states have imposed fishing regulations that re- 2001). Damming of rivers and riparian development reduced the strict or prohibit harvest. availability of vegetated, littoral habitats believed to provide suit- Despite increased interest in managing alligator gar popula- able spawning areas (Pringle et al. 2000, O’Connell et al. 2007). tions, fisheries managers know relatively little about population Currently, of the 14 states known to historically support popula- dynamics and biology of this species compared to other fishes. tions of alligator gar, the species is considered extremely rare or While current regulations are based on the best available science, extirpated from six (Jelks et al. 2008). Abundance of alligator gar in data needed to properly manage the species are lacking. Informa-

2015 JSAFWA 57 Population Dynamics of Alligator Gar in Texas Reservoir Binion et al. 58 tion on population dynamics, life history, and exploitation is need- lected during 2011 through 2013 were tagged with an external ed to better manage alligator gar stocks. Therefore, we began to T-bar anchor tag (model FD-94; Floy Tag, Seattle, Washington) at address these data needs in Choke Canyon Reservoir, Texas. Our the lateral base of the dorsal fin and a passive integrated transpon- objectives were to: 1) estimate adult abundance, 2) quantify exploi- der (PIT; model HPT9; Biomark, Boise, Idaho) at the posterior base tation, and 3) describe age structure, size structure, and length- of the dorsal fin, measured (TL), and scanned for previous marks. weight relationships. The results of our study will provide impor- To facilitate angler reporting of tagged fish, T-bar tags included tant information for the management of alligator gar populations TPWD contact information. Weight was also recorded for a sub- in Texas and throughout its distribution. sample of fish (up to 10 per 100-mm TL group) using a 100-kg digi- tal scale. To minimize the potential for immediate recapture, fish Methods were transported approximately 200 m away from the sampling site Study Site following processing and released. Incidental alligator gar mortali- Choke Canyon Reservoir is a 10,517-ha impoundment of the ties were recorded, measured (TL), and otoliths extracted for age Frio River located in McMullen and Live Oak counties, Texas. The and growth analyses. Gender of sacrificed fish and incidental mor- reservoir was created in 1982 to provide water storage and recre- talities was recorded based on internal examination of the gonads ational opportunities. Maximum depth of the reservoir is about as described by Ferrara and Irwin (2001). 29 m, with annual water-level fluctuations ranging from 3 to 4.5 m (Texas Parks and Wildlife Department [TPWD], unpublished Data Analyses data). The reservoir supports fisheries for largemouth bass (Mi- Population Abundance. Adult alligator gar abundance was cropterus salmoides) and catfishes (Ictaluridae), and is a popular estimated using the POPAN formulation (Schwarz and Arnason destination among bow anglers. Alligator gar are currently man- 1996) of the Jolly-Seber model in Program MARK (Cooch and aged with a one-fish daily bag in all Texas waters. White 2014). We used Jolly-Seber because the data best met the assumptions of an open population model (i.e., the population was Fish Collections demographically open during the study period). We considered We used multifilament gill nets to collect alligator gar annually each year of the mark-recapture study a sampling occasion (n = 3). during April through August from 2008 through 2013. Gill nets We limited our inference to the adult population given the gill- were 61-m long, 3-m deep, and constructed of 19-mm foam-core net mesh sizes used selected for fish ≥ 1100 mm (Figure 1), which float line, #21 black-twine mesh, and a 9-kg lead line. Typically, corresponds to the TL at the onset of reproductive maturity (Fer- one to three individual nets with bar-mesh sizes ranging from 89 rara 2001). The abundance estimate was then used to estimate fish to 128 mm were deployed about each sampling site. Sampling sites density, expressed as the population estimate divided by the reser- ranged in depth from 2 to 4 m, and were concentrated in tributary voir surface area at full pool. Annual mean catch-per-unit-effort arms, on main-lake coves, and on main-lake flats, primarily in the (CPUE; fish h-1) was calculated in 2011–2013 using the mean of upper half of the reservoir. All sampling sites were subjectively selected and concentrated in areas where alligator gar surface activity was observed. One or two additional nets with bar-mesh sizes of 140 and 152 mm were also set when catch rates were low (e.g., < 1 fish h–1) in attempt to increase catch. Each net was fished so that the float line remained visible on the water surface with four to five buoy-style PVC floats attached. Alligator gar entangled in the nets commonly submersed the float line or floats, facilitating visual -ob servation of fish capture. Gill nets were monitored continuously to allow for rapid removal, processing, and release (if applicable) of captured fish. Upon indication of capture, the net was lifted, fish were removed, and the net reset. Fish collections from 2008 through 2010 occurred with minimal fishing effort as alligator gar were collected solely for age and growth analysis; fish were measured for total length (TL), Figure 1. Total length distribution of alligator gar collected in Choke Canyon Reser- weighed (kg), sacrificed, and sagittal otoliths extracted. Fish col- voir, Texas, adjusted for gear selectivity.

2015 JSAFWA Population Dynamics of Alligator Gar in Texas Reservoir Binion et al. 59 the ratios approach (Pollock et al. 1994). Fishing effort was not recorded in years 2008–2010. Results A total of 754 alligator gar (total length [TL] range, 678 to Exploitation. Voluntary angler tag return data was used to 2275 mm; Figure 1) were collected during the study period. From calculate annual exploitation rates by dividing the number of tags 2011 through 2013, 656 fish were tagged and released as part of the returned by the total number of tagged fish at large. We assumed mark-recapture study. Age and growth analyses were conducted tag loss to be negligible (Buckmeier and Reeves 2012). Annual es- for a total of 98 fish. Gender could only be determined for 59 of the timates were adjusted for observed handling mortality (< 3% an- aged individuals (non-sexed individuals recorded as immature or nually). Because tag reporting rates were unknown, we estimated indeterminate). The female to male sex ratio was 1:4% (12 females, exploitation over a range of possible tag reporting rates consistent 47 males). with the literature (i.e. 20 to 80%; Miranda et al 2002, Meyer et Sixteen previously marked individuals were recaptured during al. 2012). To facilitate voluntary reporting of tagged alligator gar the study, conferring a population estimate of 5437 (95% confi- catches, notices were placed at all boat access points and in local dence limits = 3215 to 9195) individuals. All recaptured individu- and online media. als had retained both the T-bar anchor and PIT tags. Adult alliga- Alligator gar harvest data during nighttime bowfishing tourna- tor gar density was estimated at 0.5 fish ha–1 at conservation pool ments provided a means to estimate maximum harvest rates, per (95% CI, 0.3 to 0.9 fish ha–1). Catch-per-unit-effort of alligator gar unit effort, by bowfishers. We attended all known bowfishing tour- varied (range: 0.73 – 4.93 fish hour –1) by year and increased over naments held at Choke Canyon Reservoir during the study period the study period (Table 1). and recorded the number of participants, tournament length (h), A total of five tagged fish were reported by anglers during the and number of alligator gar harvested. Otoliths of harvested fish study period. Four fish were harvested by bow anglers during July were also collected to increase sample sizes for age and growth and August of 2012 and 2013, a fifth was reported caught-and- analyses. We estimated bowfishing harvest rates by dividing the released on a jugline in May of 2012. Annual exploitation was total number of alligator gar harvested by the total effort for each low during the study period, varying from 0 in 2011 to as high as tournament. 2.3% (95% CI, 1.4% to 3.2%) in 2013, assuming a non-reporting rate of 20% (Figure 2). Two night bowfishing tournaments were Age and Growth. Otoliths were processed and aged as de- held at Choke Canyon Reservoir during April and June of 2011. In scribed by Buckmeier et al. (2012). Age was assigned for each fish each tournament, 2 alligator gar were harvested (TL range, 490 to independently by two readers without knowledge of fish TL; dis- 670 mm) at rates of 0.01 and 0.02 fish angler hour –1, respectively. agreements between readers were reconciled with a subsequent Alligator gar ages ranged from 0 (i.e., young-of-year) to 27 years concert read. Total length and age-at-capture data were used to (Figure 3). The von Bertalanffy growth model indicated rapid construct a von Bertalanffy growth model for the population. Dif- growth in length through age-5 with growth slowing significantly ferences in TL-at-age among male and female fish were assessed after that (Figure 3). Sex-specific, TL-at-age data was available for using Analysis of Covariance (ANCOVA) with gender as the co- both male and female fish 9 year of age and older. Total length-at- variate. Gender specific TL-at-age data was truncated to include age significantly differed between male and female fish (ANCO- only the age class range for which TL-at-age data for both male and VA; F = 190.84, df = 1, P < 0.0001). Intercept values indicated that female fish existed. We also included the interaction of sex and age TL of female fish was about 278 mm greater than male fish of the in the model to test for differences in regression slopes between same age. The interaction term was not significant (F = 0.76, df = 1, the sexes. We further described alligator gar growth as a function of weight, expressed as the standard weight-length power function Table 1. Total catch, effort (h), and mean CPUE (fish–1 h ) of alligator gar (Pope and Kruse 2007). All statistical analyses were conducted us- collected from 2008–2013. Catch-per-unit-effort was calculated using the ing SAS Enterprise Guide 4.3 (SAS Institute 2014) and considered mean of the ratios approach. Standard deviation in parenthesis. significant atP ≤ 0.05. To obtain an accurate size composition, al- Year Total catch Total effort Mean CPUE ligator gar size structure data were corrected for size-related gear 2008 1 biases according to methods described by Millar and Fryer (1999). 2009 23 2010 48 2011 178 771 0.73 (2.3) 2012 293 408 2.68 (8.1) 2013 211 105 4.93 (11.9)

2015 JSAFWA Population Dynamics of Alligator Gar in Texas Reservoir Binion et al. 60

Figure 3. Length-at-age and fitted von Bertalanffy growth equationt ( = age in Figure 2. Annual exploitation estimates of alligator gar in Choke Canyon Reservoir, years; Lt = total length at age t; Panel A) and gender specific length-at-age (Panel B) Texas, during 2012 and 2013. Error bars represent 95% confidence limits. for alligator gar in Choke Canyon Reservoir, Texas.

P = 0.3887), indicating no difference in the rate of growth between al. 2012); all of these factors are affected by sampling design and male and female fish over the age range examined. The weight- gear specifications. Although gill nets appear to provide an effi- length relationship was significant r( 2 = 0.99, P < 0.0001); growth cient means to collect alligator gar, standardization of gear types, in weight, per unit length, was greatest in fish over 1700 mm TL. sampling methodologies, and reporting metrics among studies are critical for future comparisons among systems. Discussion Annual exploitation of alligator gar in Choke Canyon Reservoir Previous studies have used gill nets to collect alligator gar was low and similar to rates in the upper Trinity River, Texas, in (e.g., Ferrara 2001, García de Leόn et al. 2001, Brinkman 2003, 2008 and 2009 (2.8% and 3.7%, respectively; TPWD, unpublished O’Connell et al. 2007); however, differences in net specifications, data). Our highest estimate of exploitation in 2013 (3.2%, upper sampling methods, and data reporting confounded our ability to 95th percentile based on reporting response rate of 20%) was be- make meaningful comparisons of catch rates among systems. For low current TPWD management objectives (< 5%) and recom- example, O’Connell et al. (2007) used gill nets (specifications un- mended target levels of exploitation (< 10%) for other fishes with disclosed) at fixed sampling stations to collect alligator gar in Lake life histories similar to alligator gar (Walters and Pearse 1996, Cod- Pontchartrain, Louisiana, and reported catch rates as fish year –1. ling et al. 2005). The length-frequency distribution of alligator gar Ferrara (2001) and Brinkman (2003) used multifilament gill nets in Choke Canyon Reservoir provided additional evidence for low with 64-, 76-, and 127-mm bar mesh, which differed from those exploitation, as well as the potential for trophy management. Fer- employed in our study. García de Leόn et al. (2001) reported gill- rara (2001) estimated the mean TL of the alligator gar population net catches of alligator gar in Vicente Guerrero Reservoir, Mexico, to be 1518 mm in the Mobile-Tensaw Delta, Alabama, which was as fish day –1. Passive gears, such as gill nets, must be set in areas greater than populations in Lake Pontchartrain (1239 mm) and the where target fish are located, fish must encounter the net, and be Sabine National Wildlife Refuge, Louisiana (SNWR; 1241 mm). of a body size vulnerable to entanglement (Holt 1957, Hubert et The author suggested that these contrasting size structures may

2015 JSAFWA Population Dynamics of Alligator Gar in Texas Reservoir Binion et al. 61 have been due to higher exploitation of alligator gar in Lake Pon- which corresponds to the TL associated with the onset of repro- tchartrain and the SNRW. Mean TL of alligator gar in Choke Can- ductive maturity (Ferrara 2001). Growth in weight was greatest yon Reservoir (1535 mm) was comparable to that reported for the among older, larger age classes, particularly as fish approached Mobile-Tensaw Delta population of alligator gar, suggesting simi- trophy sizes. Although sample sizes were limited, our data sug- lar fishery characteristics in Choke Canyon Reservoir. Gabelhouse gested sex-specific differences in growth metrics for alligator gar; (1984) recommended relative stock density trophy (RSD-T) values females were consistently larger than males. Ferrara (2001) report- of up to 5% when trophy-sized fish are desired. In addition, al- ed similar sex-specific differences in growth. These results suggest ligator gar 1800 mm TL and greater, considered by most anglers potential for increased vulnerability of female fish to harvest when to represent trophy size classes (TPWD, unpublished data), com- compared to males, especially during the spawning season when prised about 5% of our sample. Our exploitation estimates, cou- females are in the shallow, littoral areas of a reservoir. pled with the size structure data, suggest that current exploitation In addition to sex-specific differences in growth, our results of alligator gar in Choke Canyon Reservoir is likely low enough to indicated a population dominated by male fish. It is unknown sustain a trophy fishery, if that management objective is desired. whether our observation is a natural occurrence in the popula- Documented catch and harvest occurred only during spring tion or the result of differential mortality among the sexes, gender- and summer months (i.e., May through August), periods when al- specific habitat use, or sample bias. Ferrara (2001) reported sex ligator gar may be most vulnerable to angler catch or exploitation. ratios of three alligator gar populations did not significantly dif- Spawning alligator gar are known to congregate during spring and fer from 1:1; however, male dominated populations have been re- early summer in shallow, littoral areas associated with vegetation ported for lepisosteids (e.g., Tyler and Granger 1984, Johnson and (Mendoza Alfaro et al. 2008), and can become highly abundant Noltie 1997, Ferrrara 2001). Hoenig and Hewitt (2005) attributed in specific areas for extended periods of time (Scarnecchia 1992). reduced female to male sex ratios to differential mortality experi- This behavior, coupled with increased activity and aerial breath- enced by female lumpfish Cyclopterus( lumpus) in a commercially ing during warm-water periods (Winston 1967, Saksena 1975), exploited population off the Newfoundland Coast. Faster growth suggests the potential for increased visibility to anglers and po- and greater maximum size of female alligator gar may result in tentially greater catch rates during these periods. Further research selection for female fish in recreational fisheries. While these -re is needed to quantify seasonal trends in alligator gar angler catch sults should be viewed with caution due to limited sample size, and harvest. disproportionate mortality experienced by female alligator gar Harvest rates of alligator gar during bow tournaments on could have significant effects on population reproductive capacity; Choke Canyon Reservoir in 2011 indicated that the average bow assumptions of proportional abundance between the sexes in pop- angler may harvest one alligator gar for every 50 to 100 hours of ulation models may underestimate simulated effects. Future stud- effort. Bennett and Bonds (2012) reported similar bowfishing har- ies of alligator gar should consider the collection of gender data to vest rates in the Trinity River, Texas (range, 0.016–0.023 per angler further understand the relative abundance of male and female fish hour; n = 9). Nighttime bowfishing tournament harvest rates likely in populations. represent maximum rates as tournament results are commonly The application and use of age-length keys allow fisheries man- based on the number, size, and diversity of fish harvested. There- agers to utilize a sub-sample of aged individuals to estimate age fore, non-tournament harvest of alligator gar is likely to be lower. distribution by assigning ages to fish in a specified length interval These results, coupled with the low exploitation observed during (Ketchen 1949, Isermann and Knight 2005); which can be further our study, suggest that bowfishing likely has little population-level used to construct age-specific models of mortality and survivor- impact on alligator gar at current participation rates. Additional ship (Isely and Grabowski 2007). However, considerable variabil- information characterizing both tournament and non-tournament ity in length about age could bias estimates of age composition angling effort, as well as trends in popularity of the sport, are need- limiting the utility of this approach. We observed considerable ed for objective-based management of stocks. variability and overlap in total lengths within age-class for alliga- Growth in length of fishes is commonly known to decline with tor gar during our study and felt application of an age-length key to age, whereas growth in weight commonly increases. Rapid growth estimate age structure and total annual mortality was inappropri- in length among early life-history stages improves survival by both ate. Fisheries managers should consider variation in lengths within reducing vulnerability to predators and increasing the diversity of age-classes when applying use of age-length keys for age composi- available prey (Wootton 1998). Reduced growth in length of alliga- tion and mortality estimation. tor gar in our study was observed as fish reached about 1100 mm, Our results provide the first published estimates of population

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